This invention relates to a Constant Angular Velocity (CAV) type optical disk and a recording/reproducing method therefor, and more particularly to a CAV optical disk using a sampled servo method, and a method for permitting information to be recorded onto such a CAV optical disk at a double recording density and a method for reproducing such a double density recorded CAV optical disk.
Heretofore, as a recording format of a write once type CAV optical disk, a sampled servo method or system is known. An example of such a recording format of the write once type optical disk of the sampled servo system is shown in FIG. 2. In the write once type optical disk of the sampled servo system, no pregroove (guide groove) is provided on a recording film of the write once type optical disk, and servo fields are preformated at 1376 portions in one track. This configuration will enable the optical disk of such a recording format so as to have the ability to generate clock signals for a tracking error or a recording/reproducing operation, etc. by sampling.
As shown in FIG. 2, in a program area PA of a write once type optical disk DK, spiral signal tracks are extended from the inner circumferential side to the outer circumferential side of the write once optical disk DK. One track is divided into 32 sectors. Each sector is comprised of 43 segments, and each segment is comprised of 18 bytes. At a first segment #0 of one sector, a sector synchronizing signal S.sub.sync (2bits) for taking synchronization every sector and a sector address S.sub.ADR (16bits) for indicating an address of that sector are preformated. The preformating is carried out in the process of mastering of the write once type optical disk. Each of segments #1 to #42 is comprised of a field of 18 bytes in total of a servo field Fs of 2 bytes and a data field FD of 16 bytes.
The recording format of the servo field Fs is shown in FIG. 3. The servo field Fs of 2 bytes is divided into a servo byte #1 of one byte and a servo byte #2 of one byte. A first wobble pit P.sub.w1 and a second wobble pit P.sub.w2 are preformated at the third bit position of the servo byte #1 and the eighth pit position, respectively. As shown in the figure, in the case of 16 tracks (A), the first wobble pit P.sub.w1 is located at the position of the third pit as indicated by P.sub.w1A, but when the track position shifts to the position of 16 tracks (B), the position of the first wobble pit P.sub.w1 shifts to the fourth bit as indicated by P.sub.w1B. As the result of the fact that the position of the first wobble pit P.sub.w1 is switched every 16 tracks in this way, the number of crossing tracks being searched can be precisely detected.
The first and second wobble pits P.sub.w1 and P.sub.w2 are arranged in a manner that they are shifted by 1/4 of the track pitch in the left and right directions of tracing (radial direction of the write once optical disk DK) with the track center TC being as a center. Thus, there is provided an arrangement adapted for detecting a tracking error on the basis of a difference between a return light quantity at the first wobble pit P.sub.w1 and a return light quantity at the second wobble pit P.sub.w2. At the twelfth bit of the servo byte #2, a synchronizing pit P.sub.SYNC is preformated. Between the second wobble pit P.sub.w2 and the synchronizing pit P.sub.SYNC, there exists a portion of a 19 channel clock length, and this portion is mirror-finished. For a time period during which this portion is subjected to tracking, clocks of 19 channels are counted to take synchronization. At this synchronization detecting period, a focusing error detection is also carried out. A tracking signal ST.sub.1 (ST.sub.1A or ST.sub.1B) and a sector synchronizing signal S.sub.sync obtained by reading the above mentioned servo field Fs by using a laser beam are shown in FIG. 3.
A method of detecting a tracking error by wobble pits will now be described with reference to FIG. 4. Reference symbol A indicates the case where a reading beam is passed on the center axis (track center axis) of a pair of wobble pits P.sub.w1 and P.sub.w2, and a RF signal in that case is represented as SA. In the case where the reading beam is passed through the portion in the vicinity of pits, a quantity of a reflected light is small by diffractive action of light and that portion becomes dark. Further, when the reading beam is passed immediately above the synchronizing pit P.sub.SYNC as shown, the portion becomes most dark. Reference symbol B indicates the case where the reading beam is passed through the portion on the inner circumferential side of the track center axis, and a RF signal at that time is represented as SB. In this case, because the reading beam is passed immediately above the wobble pit P.sub.w1, the dark portion by the wobble pit P.sub.w1 has a degree of darkness greter than that of the dark portion by the wobble pit P.sub.w2. In addition, reference symbol C shows the case where the reading beam is passed through the portion on the outer circumferential side of the track center axis. A.sub.RF signal in this case is represented as SC having a waveform opposite to that of SB.
Here, a signal value obtained by carrying out a signal sampling at the time of the wobble pit P.sub.w1 is assumed as a SAMPLE (T.sub.1), and a signal value obtained by carrying out a signal sampling at the time of the wobble pit P.sub.w2 is assumed as a SAMPLE (T.sub.2). In the case of A, a difference therebetween expressed as SAMPLE (T.sub.1)-SAMPLE (T.sub.2) is equal to zero; in the case of B, that difference takes a negative value; and in the case of C, that difference takes a positive value. Accordingly, when the expression of SAMPLE (T.sub.1)-SAMPLE (T.sub.2)=TE is employed, it is possible to utilize TE as a tracking error signal.
In accordance with the above described conventional sampled servo system, an approach is employed to form, in advance, wobble pits P.sub.w1 and P.sub.w2 for servo control and a synchronizing pit P.sub.SYNC on an optical disk (prepits) to provide various information for servo control such as a tracking error signal, etc. from these pit trains.
However, the track pitch width determining a recording density of an optical disk could not be held down to a predetermined value by the relationship between the track pitch width and a spot width of a laser beam. Namely, as shown in FIG. 5(A), a laser beam spot diameter BL ordinarily used is about 1 to 1.6 .mu.m (1.2 .mu.m in average) although it depends upon its wavelength, and the minimum pit width L which can be read by that beam spot diameter is about 0.4 .mu.m. Accordingly, there holds the relationship expressed as BL=3L. In the case of reading information, a laser beam reflected at the portion of the signal pit PT is diffracted by pits, so a quantity of a light returning to the optical pickup becomes small. Thus, this portion can be grasped or considered as a dark portion. In contrast, since the intermediate portion between signal pits PT is mirror-finished, a laser light is entirely reflected thereat, resulting in much quantity of a return light. Thus, that portion can be grasped as a bright portion. In order to accurately read servo information, it is necessary to read such bright and dark portions without an error. To realize this, however, a track pitch width Tp of about 1.6 .mu.m (corresponding to 4L) was conventionally required.
In order to improve the recording density of the optical disk DK, it is necessary to further shorten the wavelength thus to reduce the dimension of pit, or to further shorten the track pitch width. In the present state, however, there is a limit in reduction of a laser beam. On the other hand, when consideration is made in connection with the case where the track pitch width Tp is reduced to about 2L (about 0.8 .mu.m) as shown in FIG. 5(B) or 5(C), a difference between a light quantity in the case of FIG. 5(B) corresponding to the on-track state where a laser beam center is positioned on the track axis center and a light quantity in the case of FIG. 5(C) corresponding to the off-track state where the laser beam center deviates from the track axis center becomes small, resulting in the possibility that servo control cannot be precisely conducted.